On Tue, 2021-09-14 at 12:52 +0200, Emanuele Giuseppe Esposito wrote:

quoted

I would do it this way:

struct svm_nested_state {
         ...
	/* cached fields from the vmcb12 */
	struct  vmcb_control_area_cached ctl;
	struct  vmcb_save_area_cached save;
         ...
};

The only thing that requires a little bit of additional work when 
applying this is svm_get_nested_state() (and theoretically 
svm_set_nested_state(), in option 2). In this function, nested.ctl is 
copied in user_vmcb->control. But now nested.ctl is not anymore a 
vmcb_control_area, so the sizes differ.

There are 2 options here:
1) copy nested.ctl into a full vmcb_control_area, and copy it to user 
space without modifying the API. The advantage is that the API is left 
intact, but an additional copy is required.

Thankfully there KVM_GET_NESTED_STATE is not performance critical at all,
so a copy isn't that big problem, other that it is a bit ugly.
Ugh..

2) modify KVM_GET_NESTED_STATE and KVM_SET_NESTED_STATE to handle 
vmcb_control_area_cached. Advantage is that there is a lightweight copy 
+ the benefits explained by you in the previous email (no unset field).

That would break the KVM_GET_NESTED_STATE ABI without a very good reason,
especially since some of the currently unused fields in the ctl (there
are I think very few of them), might became used later on, needing
to break the ABI again.

Best regards,
	Maxim Levitsky

I am not sure which one is the preferred way here.

Thank you,
Emanuele